<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>

        * {
            box-sizing: border-box;
        }

        body {
            height: 100vh;
            margin: 0;
        }

        header {
            height: 40px;
            background: skyblue;
            color: white;
        }

        main {
            min-height: calc(100vh - 40px - 100px);
        }

        footer {
            height: 100px;
            background: #333;
            color: white;
        }
    </style>
</head>
<body>
<header>这是头部</header>
<main>这是内容</main>
<footer>这是底部</footer>
</body>
</html>
